1. Field of the Invention
The present invention relates to a shutter of electromagnetic actuator driving type for cameras.
2. Description of Related Art
In a conventional shutter of electromagnetic actuator driving type for cameras, in cases where the opening speed of the shutter blades thereof is required to be mechanically changed, it has been practiced to vary a spring force either by selecting one of various springs which have different spring forces or by changing a hooking position of a torsion spring.
Also, in the case of a camera shutter of the kind having the shutter blades arranged to be opened directly by an electromagnetic actuator and closed by a restoring force of a spring, as disclosed in Japanese Laid-Open Utility Model Application No. HEI 5-59435, the spring is a torsion spring and the restoring force of the torsion spring is arranged to be adjustable by rotating a mount part on which the base of the torsion spring is mounted.
However, in the first-mentioned conventional example, since the opening speed of the shutter blades is adjusted by selecting one of springs of different spring forces or, with a torsion spring having an unstable spring force used for the shutter, by varying the hooked position of the torsion spring, such an arrangement hardly permits fine adjustment, because the selection of the spring force for adjustment of the shutter-blade opening speed is made from among spring forces which are set not continuously but stepwise. The shutter-blade opening speed thus cannot be accurately adjusted
Further, as for the second-mentioned device disclosed in Japanese Laid-Open Utility Model Application No. HEI 5-59435, the device is for adjustment of the shutter-blade closing speed of a camera shutter and not for the shutter-blade opening speed. Also, since a torsion spring is used as a shutter-blade closing spring and the shutter-blade closing speed is adjusted by rotating (turning) the base mounting part of the torsion spring in such a way as to adjust the restoring force of the torsion spring, it is, however, difficult to linearly adjust the restoring force of the torsion spring by such an arrangement. In addition, another problem arises in that, since the torsion spring is arranged to be hooked directly on a hole provided in a shutter blade, such a spring hooking arrangement, however, tends to damage the hole provided in the shutter blade.